1. Field of the Invention
This invention relates to a method for delignifying and/or bleaching a lignocellulosic material, and more particularly, to a method for improving the performance of chlorine dioxide in the process of bleaching lignocellulosic pulp.
2. Description of the Prior Art
Pulp mills are aggressively moving to elemental chlorine-free bleaching to address environmental and market issues. The move to chlorine dioxide virtually eliminates the formation of poly-chlorinated phenolic compounds, and substantially reduces the contributions of adsorbable organic halides (AOX), colour and chloroform in the effluent. Drawbacks of chlorine dioxide include the high cost of increased chlorine dioxide generation and the poorer bleaching performance of chlorine dioxide in the first stage (delignification) of the bleaching process when chlorine is replaced entirely by chlorine dioxide. The need for more oxidation equivalents increases the overall bleaching cost to reach a target kappa number and/or brightness.
Over the years various modifications and improvements to the basic chlorine dioxide process have been described in the technical literature. These efforts have addressed the optimization of the bleaching conditions and the optimization of the bleaching sequence configurations. However, little work has been done on enhancing chlorine dioxide bleaching through the use of additives. Dimethylsulfoxide, sulfamic acid and hydrogen peroxide have been tested as additives in chlorine dioxide bleaching to reduce the formation of AOX, but these additives also reduced the delignification efficiency of chlorine dioxide and increased bleaching cost to a varying degree [Y. Ni, G. J. Kubes, and A. R. P. van Heiningen, J. Pulp Paper Sci., 1994, 20(4): J103-106; M. J. Joncourt, P. Froment, D. Lachenal, and C. Chirat, International Pulping Conf., Chicago, Oct. 1-5, 1995, p 149-152; M. J. Joncourt, G. Mortha and D. Lachenal, International Symp. Wood Pulping Chemistry, Montreal, Jun. 9-12, 1997, p J6-1 to J6-5; B.-H. Yoon and L.-J. Wang, International Pulp Bleaching Conf., Helsinki, Jun. 1-5, 1998, p 407-412]. It is known that a certain proportion of chlorine dioxide is converted to chlorite during chlorine dioxide bleaching, which may represent a loss of oxidizing power of chlorine dioxide [W. H. Rapson and C. B. Anderson, CPPA Trans. Tech., Sect., 1977, 3(2):Tr52-55 and Tappi J., 1978, 61(10):97-99; I. Wartiovaara, Paperi ja Puu-Papper o. Tra, 1982, 64(9): 534-545 and 64(10): 581-584]. A two-step high-pH/low-pH method for chlorine dioxide bleaching has been proposed and tested as a means to improve the performance of the chlorine dioxide bleaching process [G. E. Seger, H. Jameel and H.-m. Chiang, Tappi J., 1992, 75(7):174-180]. The working hypothesis of the two-step method is that chlorate formation can be reduced at the higher pH conditions, and the chlorite formed in the first high-pH step can become reactive in the second low-pH step [G. E. Seger, H. Jameel and H.-m. Chiang, Tappi J., 1992, 75(7):174-180; B.-H. Yoon and L.-J. Wang, International Pulp Bleaching Conf., Helsinki, Jun. 1-5, 1998, p 407-412]. Iron has been tested as an additive in chlorine dioxide bleaching to regenerate chlorine dioxide from chlorite [M. J. Joncourt, G. Mortha and D. Lachenal, International Symp. Wood Pulping Chemistry, Montreal, Jun. 9-12, 1997, p J6-1 to J6-5]. It was found, however, that iron also reacted with chlorine dioxide and reduced its delignification efficiency. Moreover, a significant loss of pulp viscosity was observed.